1. Field of the Invention
Aspects of the invention relate to certain methods and apparatuses by which data is written to a medium.
2. Description of Related Art and General Background
Certain memory devices, e.g., tape drives perform read-after-write verification in order to guarantee that the data written to the medium can be recovered sometime in the future upon read back. During the write process, the written data is read back, and the read back data is checked to insure that the data was written correctly and that it can be recovered at a later time. The data check typically consists of calculating an Error Detection Code (EDC) on the read data and comparing the result with the value written onto the medium. A write error is declared when the EDC written on the medium does not match the calculated EDC. If a write error is declared, the suspect data is typically rewritten on the medium. Most EDCs simply detect whether an error occurred, not the severity of the error. Thus a single bit in error can cause data blocks to be re-written.
U.S. Pat. No. 5,594,599 discloses an apparatus that verifies proper operation of a recording and reproducing apparatus by comparing, after a suitable delay, EDCs calculated for compressed data prior to being recorded on a medium. The apparatus uses EDCs calculated for the corresponding compressed data reproduced from the medium, and thereby reduces the amount of:delay memory needed to temporarily store the error detection codes calculated prior to recording of the data. The apparatus, however, does not provide a mechanism whereby the actual read data is compared to the actual write data to detect a severity of the error.
Current digital, linear tape/super digital linear tape (DLT/SDLT) Read-After-Write (RAW) strategies use a 64 bit physical block cyclic redundancy check (CRC64) as the criteria for whether a block has been successfully written to a medium. If the CRC64 read from the medium does not match the value computed from the read data, then the block is deemed to be in error on the medium and should be rewritten. The CRC64 is capable of detecting read back errors, but cannot determine the severity of the error. The current RAW strategy requires that any block written which has a RAW CRC error should be rewritten onto the medium.
For next generation products, aggressive improvements in track width, track pitch, bit density and reduced signal-to-noise ratio (SNR) appear to show that the current RAW strategy will be inefficient and ineffective. Measured channel error statistics have shown that if the current RAW strategy is maintained, then the amount of rewrites will increase dramatically, impacting media capacity and data rate. Thus a new RAW strategy is needed.
An embodiment of the present invention receives a bit stream of information from memory, and saves the information in a storage area. The information is then written to a medium, after which it is read from the medium and compared to the information saved in the storage area. A number of miscompares is counted between the information read from the medium and the information saved in the storage area, and the information is rewritten to the medium when more than a predetermined amount of miscompares has occurred.